1. Field of the Invention
This invention relates to viscous shear couplings. Viscous shear couplings in accordance with the invention have particular, but not exclusive, application in the drive transmissions of motor vehicles. The invention is also concerned with four-wheel drive vehicles including such viscous shear couplings.
2. Description of Prior Art
An example of a viscous shear coupling, which is described in GB-PS No. 1357106 (and in the corresponding U.S. Pat. No. 3,760,922 issued Sept. 25th, 1973), comprises a housing part; a hub part within the housing part, the parts being relatively rotatable about a common axis; a viscous liquid in the housing part; and first and second sets of annular plates in the housing part, the plates of the first set being rotationally fast with the housing part and being interleaved with the plates of the second set which are rotationally fast with the hub part.
Another example of a viscous coupling is shown in European No. PS-0068309 (and in the corresponding U.S. Pat. No. 4,721,010 issued Jan. 26th, 1988). In this coupling there is a housing part and first and second hub parts within the housing part, all the parts being relatively rotatable about a common axis. There are first, second and third sets of annular plates in the housing part, the plates of the first set being rotatable with the housing part and being interleaved with the plates of the second and third sets which are rotatable with the first and second hub parts respectively. There is a viscous liquid in the housing part.
Torque is transmitted between the housing part and the hub part(s) of the coupling due to shearing forces in the viscous liquid caused by relative rotation between adjacent plates. Viscous shear couplings are used, inter alia, in the drive transmissions of motor vehicles having four-wheel drive of the type known as a viscous transmission. In such a transmission there is a permanently driven axle and a viscous shear coupling is disposed in the drive to the wheels of a second axle thereof. For example, the vehicle may have a forwardly mounted engine, gearbox, and conventional drive arrangement to the vehicle front wheels, with an additional power output from the gearbox leading to a longitudinal drive line and the rear axle of the vehicle, the viscous shear coupling being disposed in such longitudinal drive line. As long as the wheels of the permanently driven front axle, due to a sufficiently high adhesion between tires and road surface, drive the vehicle with no or substantially no slip, there is practically no speed difference between the two parts of the viscous shear coupling, so that no torque is transmitted to the rear wheels. However, if one of the permanently driven front wheels spins due to lack of adhesion between tire and road surface, a speed difference occurs across the viscous shear coupling so that there is relative rotation between the parts thereof and torque is transmitted to the rear wheels through the coupling. The rear wheels then participate in driving the vehicle.
In another example of a viscous transmission a viscous shear coupling as described in European No. PS-0068309 is used in the rear axle and serves both as a drive unit and as a rear-axle inter-wheel differential gear.
With such a viscous transmission, problems can occur under braking. If the vehicle is braked sharply or on a slippery surface so that the front wheels of the vehicle are locked, the action of the viscous shear coupling will be such as to attempt also to lock the rear wheels and this is undesirable since, if the rear wheels lock, lateral stability of the vehicle is impaired. Moreover, if an anti-lock braking system is fitted to the vehicle, operation of the viscous coupling could, when the front wheels tend to lock, obstruct the operation of the braking system.
It has been proposed to include a one-way clutch unit in a viscous transmission so that no torque is transmitted through the longitudinal drive line via the viscous shear coupling when the rear wheels overrun the front wheels (see for example U.S. Pat. No. 4,605,087 issued Aug. 12th, 1986). Clearly to provide such an additional clutch unit is expensive and adds weight and complexity to the vehicle.